I'm not that familiar with dendrobiums and the one dendrobium I have has a completely unique culture from other dendrobiums, but hopefully something I have to say will help you out and maybe some other folks will see your post and chime in too.
I think the plant might be overwatered. The medium looks like it's VERY wet and there seems to be some mold growing at the base of the cane. I could be wrong, I don't know how much larger dendrobiums need to be watered or what sort of conditions they like. Your medium looks like it's staying constantly wet though and even orchids that like to have wet feet, have to dry out some between waterings.
How often are you watering it? Daily?
If so... then the most likely culprit here is some kind of rot. Orchids are very susceptible to rot and the conditions they grow in, in pots, create a perfect environment for rot to set in. I would cut back on the watering for now.
What is it potted in? I'm having a hard time telling from your photos. It looks like its potted in dirt, but it could also be moss. If this orchid is potted in moss, the medium is way too broken down and is probably not allowing a lot of air circulation to the roots, which is part of your rot problem too. It will need to be repotted after the blooms have dropped.
All of this stuff, is stuff I think all of us are guilty of at some point when we start keeping orchids, so don't let it hold you back! Hopefully someone with more experience with dendrobiums can recommend the proper culture for this plant for you. Good luck!